A common radio architecture in integrated Radio Frequency (RF) transceivers for wireless devices for a cellular communications network utilizes direct conversion receiver(s) and direct conversion transmitter(s). In a direct conversion transmitter, baseband information to be transmitted is upconverted directly from baseband to the desired RF carrier frequency. Likewise, in a direct conversion receiver, a received RF signal is downconverted directly from a corresponding RF carrier frequency to baseband. Direct conversion provides a power optimized implementation in an integrated circuit. Direct conversion transmitters and direct conversion receivers require a good, stable Local Oscillator (LO) signal. As such, the LO signal for a direct conversion transmitter or receiver is normally generated using a Phase-Locked Loop (PLL), where a controllable LC oscillator is phase-locked to a stable Crystal Oscillator (XO).
For a wireless device operating in, e.g., Evolved Universal Terrestrial Radio Access (E-UTRA) Frequency Division Duplexing (FDD) mode, both the transmitter of the wireless device and the receiver of the wireless device are operating at the same time. This in turn means that a PLL generating the LO signal for direct upconversion in the transmitter (which is referred to herein as a transmit PLL) and the PLL generating the LO signal for direction downconversion in the receiver (which is referred to herein as a receive PLL) are enabled, or running, at the same time. Further, if the wireless device is communicating in multiple aggregated frequency bands using a Carrier Aggregation (CA) scheme, several transmit PLL(s) and/or receive PLL(s) will be enabled, or running, at the same time. Several PLLs running at the same time may also be required in other scenarios such as, e.g., when the wireless device has two simultaneous “calls” (also referred to as a dual call scenario). The two simultaneous calls may be, e.g., a voice call via a Global System for Mobile Communications (GSM) Radio Access Network (RAN) and a data call via an E-UTRA network (i.e., a 3rd Generation Partnership Program (3GPP) Long Term Evolution (LTE) RAN).
One issue that arises from multiple simultaneously enabled PLLs when integrated into a single Integrated Circuit (IC) is crosstalk between the PLLs and, in particular, crosstalk between the controlled LC oscillators of the PLLs. LC oscillators are based on inductors that are implemented using metal wires in the IC. These inductors tend to be relatively large devices in the IC and normally have a size of several hundred micrometers. When implementing two or more LC oscillators on the same IC, there will inevitably be crosstalk between the LC oscillators. The crosstalk can be both inductive and capacitive. This crosstalk can be mitigated by increasing the distance between the LC oscillators, but the distance between the LC oscillators and thus the mitigation achieved by increasing the distance between the LC oscillators is limited by the size of the IC and the number of LC oscillators in the IC. Inductive crosstalk can be somewhat mitigated by using complex inductor layouts at the expense of lowered Q-value and thus increased power consumption. Further, the amount of crosstalk is difficult to predict using simulation because the amount of crosstalk is very much dependent on metallization between the LC oscillators in the IC.
The issue of crosstalk between PLLs on an IC is illustrated in FIG. 1 and FIGS. 2A and 2B. In particular, FIG. 1 illustrates an IC 10 including two PLLs 12-1 and 12-2. As illustrated, the PLLs 12-1 and 12-2 include Controlled Oscillators (COs) 14-1 and 14-2, respectively. The COs 14-1 and 14-2 are more particularly LC oscillators. In this example, the CO 14-1 includes an inductor 16-1, a capacitor 18-1, and a pair of cross-coupled transistors 20-1 and 22-1 having cross-coupled gates and drains. Likewise, the CO 14-2 includes an inductor 16-2, a capacitor 18-2, and a pair of cross-coupled transistors 20-2 and 22-2 having cross-coupled gates and drains. Coupling (k) between the inductors 16-1 and 16-2 results in crosstalk between the two COs 14-1 and 14-2.
FIG. 2A illustrates output spectra of the PLLs 12-1 and 12-2, and more specifically of the COs 14-1 and 14-2, of FIG. 1 according to a first example in which the COs 14-1 and 14-2 are placed in close proximity to one another. An output signal (C1) of the CO 14-1 is at a first frequency (f1), and an output signal (C2) of the CO 14-2 is at a second frequency (f2). Due to the coupling (k) between the inductors 16-1 and 16-2, the output signal (C2) of the CO 14-2 results in a crosstalk signal from the CO 14-2 being injected into the CO 14-1 at the second frequency (f2). The amplitude limiting function of the CO 14-1 converts the crosstalk signal at the second frequency (f2) into a phase-modulation of the CO 14-1, which in turn results in a symmetric phase-modulated output spectra for the CO 14-1. The symmetric phase-modulated output spectra for the CO 14-1 includes modulation sideband signals (L2 and L2′) at frequencies of f1+|f1−f2|=f2 and f1−|f1−f2|, respectively, where in this example |f1−f2| is equal to a value “df.” Note that the modulation sideband signal (L2) corresponds to the crosstalk signal at the second frequency (f2). Likewise, due to the coupling (k) between the inductors 16-1 and 16-2, the output signal (C1) of the CO 14-1 results in a crosstalk signal from the CO 14-1 being injected into the CO 14-2 at the first frequency (f1). The amplitude limiting function of the CO 14-2 converts the crosstalk at the first frequency (f1) into a phase-modulation of the CO 14-2, which in turn results in a symmetric phase-modulated output spectra for the CO 14-2. The symmetric phase-modulated output spectra for the CO 14-2 includes modulation sideband signals (L1 and L1′) at frequencies of f2−|f1−f2|=f1 and f2+|f1−f2|, respectively, where again in this example |f1−f2| is equal to a value “df.” Note that the modulation sideband signal (L1) corresponds to the crosstalk signal at the first frequency (f1).
Also, the COs 14-1 and 14-2, which again are LC oscillators, act as filters to the crosstalk signals. Thus, when increasing the offset between f1 and f2 (i.e., when increasing |f1−f2|, the level of the crosstalk signals (i.e., the level of the phase-modulation sidebands) will roll off by 6 Decibels (dB) per octave. For example, as illustrated in FIG. 2B, when the offset frequency is doubled, the level of the crosstalk signals decreases by 6 dB.
In normal E-UTRA FDD mode, the duplex distance is fixed and relatively high. In this case, only two PLLs are active, and the PLL isolation requirements are normally manageable. However, when adding CA or a dual call, the number of frequency combinations substantially increases, several PLLs are active at the time, and oscillator frequencies can be placed very close to one another, which in turn results in higher levels of crosstalk signals.
Further, the crosstalk becomes particularly problematic if the modulation sidebands are placed in a position that causes receiver desensitization. For example, if the output of the transmit PLL has a modulation sideband on the receive frequency, the modulation sideband results in receiver desensitization. As another example, receive desensitization also occurs if the output of the receiver PLL includes a modulation sideband on the transmit frequency. A similar issue arises if the output of the receiver PLL includes a modulation sideband on other transmit frequencies of the wireless device such as, e.g., a Wireless Local Area Network (WLAN) transmit frequency.
European Patent Application Publication No. EP 2600544 A1, entitled “Technique for crosstalk reduction,” describes a technique for cancelling or reducing crosstalk between COs in an IC. In particular, in order to cancel or reduce crosstalk from a first PLL to a second PLL in an IC, a cancellation signal is generated at an output frequency of the first PLL (i.e., at the same frequency as the crosstalk signal from the CO of the first PLL injected into the CO of the second PLL) and injected into the CO of the second PLL. The cancellation signal is generated from the output of the first PLL. An amplitude of the cancellation signal is controlled to be substantially the same as that of the crosstalk signal, and a phase of the cancellation signal is controlled to be substantially the opposite of that of the crosstalk signal. When injected into the CO of the second PLL, the cancellation signal cancels or reduces the crosstalk signal from the first PLL.
While the technique described in EP 2600544 A1 provides good crosstalk reduction, there remains a need for systems and methods for improved crosstalk reduction.